Conventionally, there has been known a method of estimating the pressure and temperature (hereinafter referred to as “intake air pressure” and “intake air temperature,” respectively) of air in an intake passage of an internal combustion engine between a throttle valve and an intake valve thereof (hereinafter referred to as a “post-throttle intake passage”) through calculation; specifically, through application of physical laws, such as the mass conservation law, the energy conservation law, and the state equation, to the air in the post-throttle intake passage (see, for example, the pamphlet of WO2003/033897).
Specifically, in the above-mentioned document, a time-course change d(Pm/Tm)/dt in a value (intake air pressure temperature ratio) Pm/Tm obtained by dividing the intake air pressure by the intake air temperature is estimated through use of the following Expression (1), and a time-course change dPm/dt in the intake air pressure Pm is estimated through use of the following Expression (2).d(Pm/Tm)/dt=(R/Vm)·(mt−mc)  (1)dPm/dt=κ·(R/Vm)·(mt·Ta−mc·Tm)  (2)
In Expressions (1) and (2) given above, Pm represents the intake air pressure; Tm represents the intake air temperature; R represents the gas constant of air; Vm represents the volume of the post-throttle intake passage; mt represents the mass flow rate (mass per unit time) of air flowing into the post-throttle intake passage via the throttle valve; mc represents the mass flow rate (mass per unit time) of air flowing out of the post-throttle intake passage via the intake valve; κ represents the specific-heat ratio of air; Ta represents the temperature of air flowing into the post-throttle intake passage via the throttle valve (atmospheric temperature); and t represents time.
Expression (1) is derived through application of the mass conservation law and the gas state equation to air in the post-throttle intake passage. Expression (2) is derived through application of the energy conservation law and the gas state equation to the air in the post-throttle intake passage. The method of deriving these expressions is described in detail in the above-mentioned document.
The intake air pressure Pm is iteratively estimated by means of iteratively integrating, with respect to time, the value of dPm/dt obtained from Expression (2). Also, the intake air temperature Tm is iteratively calculated on the basis of the iteratively estimated intake air pressure Pm, and the intake air pressure temperature ratio Pm/Tm, which is iteratively estimated by means of iteratively integrating, with respect to time, the value of d(Pm/Tm)/dt obtained from Expression (1). As described above, in the above-mentioned document, the state of air in the post-throttle intake passage (the intake air pressure Pm and the intake air temperature Tm) are iteratively estimated by means of iteratively integrating Expressions (1) and (2) with respect to time.
Incidentally, a volume which has a substantial influence on changes in the intake air pressure Pm and the intake air temperature Tm (hereinafter referred as the “effective volume”) is used as the volume Vm of the post-throttle intake passage in Expressions (1) and (2). In general, difficulty is encountered in accurately calculating the effective volume Vm on the basis of only the geometrical shape of the post-throttle intake passage. Accordingly, in order to accurately estimate the intake air pressure Pm and the intake air temperature Tm through use of Expressions (1) and (2), a test (identification experiment) for identifying the effective volume Vm must be carried out.
In this identification experiment, the effective volume Vm is identified, through utilization of a known statistical technique, such that changes in the intake air pressure Pm and the intake air pressure temperature ratio Pm/Tm, which are obtained by iteratively integrating Expressions (1) and (2) with respect to time, approach changes in the actually measured corresponding values, respectively. Both of Expressions (1) and (2) include the term of the effective volume Vm. Therefore, the changes in the intake air pressure Pm and the intake air pressure temperature ratio Pm/Tm may vary depending on the value of the effective volume Vm. That is, it is necessary to identify the effective volume Vm, while monitoring both the changes in the intake air pressure Pm and the intake air pressure temperature ratio Pm/Tm. In addition, since both of Expressions (1) and (2) include a differential term, the degree of change in the intake air pressure Pm and the intake air pressure temperature ratio Pm/Tm in relation to a change in the value of the effective volume Vm is likely to become relatively large. As a result, there has been a problem in that the identification of the effective volume Vm is rather difficult.